The invention relates to static AC-to-DC converters such as can be used for AC or DC motor drives. The problem encountered with AC/DC converters is the generation of harmonics to the AC side caused by the rectification process. The need for reduction of AC line current harmonics drawn by the converter has become increasingly important, especially for applications incorporating emergency power sources, using diesel generators, since these have a relatively high impedance. In order to reduce the effect of harmonics, the most favorable prior art has used transformers having polygonal transformer connections combined with bridges of static switches operating on the respective phases and controlled so as to form a series of current pulses approximating the desirable sinusoidal waveform. See for instance Johannes Schaefer, Rectifier Circuits: Theory and Design/John Wiley & Sons, 1965. Increasing the pulse number and multiplying the number of phases on the AC side will improve the quality of the generated AC current. However, this is at the expense of using more transformers and of providing phase shifting transformers, a drawback, especially in lower power systems. More specifically, reference is made to U.S. Pat. Nos. 4,255,784 and 4,683,527 of John Rosa relating to a 12-pulse AC/DC converter of the hexagon type, one with two transformers, the other with a single transformer.
The object of the present invention is to use the polygon transformer technique of the prior art in such a way that the number of pulses is increased without having to increase the number of transformers to be used, while providing transformers of lower rating.